The present invention relates to a parallel interface for an external device such as a scanner and printer, and more particularly to a parallel interface for a scanner and printer with bi-directional transmission and DMA (Direct Memory Access) capabilities.
A parallel interface has been the most popular interface between a personal computer and a printer. The interface uses Strobe, Busy, and Acknowledge signals to transfer the data reliably between the printer and the computer. Table 1A shows the typical pin assignment of the parallel interface for a printer, and FIG. 1 shows the timing requirements of the interface for a computer such as the typical PC, (e.g., an IBM PC). In FIG. 1, the parallel interface timing signals are the BUSY, ACKNLG, DATA and STROBE signals.
FIG. 2 shows a block diagram of the typical implementation of a parallel interface. In FIG. 2, system 10 is connected to address decoder control circuit 12 through system address bus 32 and system control bus 34. System 10 is also connected to transceiver 14 through system data bus 30.
Address decoder control circuit 12 communicates to transceiver 14 through internal control bus 22. Also, control circuit 12 communicates to output data latch 16 through control bus 22, as well as to read/write control logic 18.
Transceiver 14 communicates with output data latch 16 and read/write control logic 18 through internal data bus 24.
Output data latch 16 is connected to parallel port connector 20 through parallel data bus 26. Similarly, parallel port connector 20 is connected to read/write control logic 18 through parallel control bus 28.
In the case of a typical PC type computer, the handshake is built in the BIOS where the software controls the necessary checking of signals. However, this software approach does not allow fast data transfer from the host to a printer. The timing requirements of FIG. 1 suggest a maximum transfer rate of 156K bytes per second while the manual suggests 1K bytes per second transfer rate.
When a large amount of data (such as data representative of an image) is involved, the communication between the host Computer and the printer becomes a bottleneck. In addition, a host computer can not receive the limited status information of a printer through few control signals because the data are transferred only from the host computer to the printer.
When image scanners were introduced, interfaces to a host computer were typically a parallel interface and RS232 interface. The parallel interface is similar to the one used for a printer, except that the direction of data movement is bi-directional.
Tables 1A and 1B summarize the printer and scanner pin assignments, respectively. In Table 1B for a scanner, pin 15 is typically assigned to control the direction of the data. When the signal is high, the data transfer from a host to a device. When it is low, the direction is reversed.
Table 2 shows the PC printer interface cable definition. Table 2 shows that the pin 15 of the host side is a major source of conflict between a scanner and a printer. The rest of the pins are covered by scanner or printer pin definitions.
The problem of data transfer rate from a host computer to a printer or plotter was addressed by IKON. IKON developed two boards which can use the DMA capability of the typical PC type computer. However, the major targets of their boards were plotters and printers. Therefore, their connector was not the standard 25 pin connector. This does not allow regular printer cable to be used. In addition, the boards have extra capability which are not required for a regular printer use. Examination of the boards showed more than 60 chips resulting in higher cost.
Ricoh Corporation has developed a parallel interface card, ISI-8, for a scanner family. This board was specially designed for scanner use and the design did not address the possible use for a printer. Because a scanner uses pin 15 for a direction control (Table 1B), this line is critical to the scanner use. The signal is controlled by a host computer side as an output signal to a device. However, in a regular printer cable, pin 15 is connected with pin 32 of a printer as an input from a device to a host computer. Therefore, the current ISI-8 interface card cannot be connected with a printer through the regular cable.